This is an extended filter basket.  The HEPA can sit on top of the filter and then the carbon material can go below it.  The HEPA filter is the same as others https://www.amazon.com/gp/product/B0782T7L6P (HEPA Filters Replacement for ILIFE V3s V3s pro V5 V5s V5s Pro V5s Pro 2 V5s Plus).  The HEPA filter is held in place by the snug fit of the housing and the model and should rest on top of the model lip as pictured.

Print main body as all wall for strength as well as the upper insert.

Lower insert should be printed with 0 top/bottom layers, 6 walls and with ~10-15% infill depending on the infill used.  I used honeycomb but your carbon material will dictate the size you need, but this should be larger to allow airflow.

Use acid-free carbon only, Nevermore material is my choice.